Microphone



I 'ROPHQ Filed Ju 2) 925 INVEN NEY Patented Nevf27, 192s.

. UNITED srAr 1PATENTOFFICE.

JOSEPH EUGENE AIKEN, F PITTSBURGH, PENNSYLVANIA, ASSIGNOR TO WESTING-HOUSE ELECTRIC & MANUFACTURING VAN IA.

COMPANY, A CORPORATION OF PENNSYL- mcnornonn.

Application filed July 2 18 25. Serial No. 41,178.

Another object of my invention is to pro-' vide a microphone in whichthe separation between the edge of the granule cup and the surface ofthe diaphragm is greater than the maximumexcursion of the diaphragm andless than the radius of the resistor granules.

r Another object of my invention is to provide a microphone structure inwhich damping means are provided whereby the excursion of the diaphragmis reduced and whereby a closer spacing between the edges of theresistor granule cups andthe 'medianposition-of the diaphragm isobtained.

Another object of my invention is to provide a preferential dampingmeans whereby high vibration frequencies in the diaphragm are moreeffectively damped than low vibration frequencies.

" Another ob'ect of my invention-is to provide a microp one systemhaving a stretched diaphragm, a preferential dampin means cooperatingtherewith, a pair of resistor elements and means for retaining theresistor v granules in place without the use of unsymmetrical closuresor padding.-

Another object of my invention is to pro- 36' vide a microphone havingsmall resistor granules arranged in a symmetrical, uniform pressuresystem.

In the construction of microphone devices,

it has been customary to embody therein a re- 'sistor element consistingof a cup filled with granules, of such material as carbon or graphite,which rested a ainst the diaphragm and were held in place by a packingof elt between the ed e of the granule cup and the surface of theiaphragm. This construction has been found to produce an unsymmetricalarrangement of the granules which does not give an accurate translationof the sound vibrations into electrical vibrations. The

failure of accuracy is probably due to random displacements in theunsymmetrical pressure system. i

My invention provides means whereby the granule mass is retained with acup having simple surfaces only and no packing felt, and whereby asimple, symmetrical distribution of the granules in the cup is obtainedand simple symmetrical pressure system provided.

' The construction provided embodies damppreferential damping effect,comprisinga pair of granule cups having simplesurfaces and sharpedges,one thereof being an integral part,0f the damping means, and resistorgranules having a radius larger than the spacing between the granulecups and thea diaphragm. By this means, I am enabled to provide a microhonedevice in which compensation is ma e for the normal departure of thereproduction curve from a straight line, wherein the occurrence ofrandom departures from a" straight line in individual resistors isminimized, and wherein the tendency of a microphone to respond morestrongly to high frequencies than to low is compensated.

Other objects and structural details of my invention will be apparentfrom the following description, when read in connection with theaccompanying drawings, wherein- Figure 1 is a view in vertical elevationof an embodiment of my invention;

Fig. 2 is a view in rear elevation of the same; and

Fig. 3 is a sectional view alongthe lines III-III of. Fig. 1.

The preferred embodiment of my improved microphone shown in the figurescomprises a frame 1,"a.clamping ring 2 and a diaphragm 3 held by itsedges therebetween. Clamping pressure is furnished screws 4. s

Stretching means are provided for the diaphragm 3,;comprisinga ring 5having rounded edges and a pressure ring 6 threaded-into the frame 1. Aback plate 7 is provided, preferably formed of insulating material, suchas hard rubber. The backplate 7 is conveniently held in place by a;second clamping ring 8, and is spaced away from the diaphragm by amicaring 9.

The back plate 7 is co inter-bored to'form a cylindrical central recess11, at the bottom of which is mounted a metal connector member 12 havinga carbon facing 14; The coning means, which are also made to provide, a,

by the I ductor members 12 and 1-1 are held in place by a threaded bolt15 having a shoulder portion 16. The recess 11 contains a mass ofgranules 17 which almost fill it. Electrical 6 connection is made to themembers 12 and 11 and to the granules by a wire or other conductorattached to the binding post 18. Holes 19 are provided near theperiphery of the back plate 7 for controlling the relative de 10 gree ofdamping of high-frequency vibrations as compared to low-frequencyvibrations.

A second granule cup 21 is provided, of insulating material or having aninsulating i5 lining (not shown). Within the granule cup 21 is mounted aconducting member 22 and a carbon or graphite facing member 23. Thegranule cup assembly is attached to a supporting bar 24 which is held ina clamp 25. 2a A bridge piece 26 is provided, attached to the frame 1 byscrews 27 and insulated therefrom'by insulating members 28 and 29. Thegranule cup 21 is filled with granules 31 in a manner similar to thefilling of the space 11 .25 in the back plate 7.

The granules 17 and 31 may conveniently be of a diameter between 0.010and 0.020 inch, and may be of a single size or of several slightlydifferent sizes.

In the assembly of the device of my invention the diaphragm 3 is. placedon the frame 1, the clamping ring 2 positioned thereon and the screws 4inserted and drawn up tightly.

The pressure ring 5 is then inserted within the annular opening of theframe 1 and the ring 6 screwed down upon it with a conve nient pressure,which, should be adequate to stretch the diaphragm almost to its elasticlimit.

The various parts of the back plate 7 and the conductor members 12, 1 1,15 and 16 are thenassembled in the obvious way. A sufficient quantity ofgranules 17 are inserted in the recess 11 almost to fill it full. Thespacingring 9, which may in of mica, having ia thickness between 0.001ad 0.002, is then laid concentrically upon the top of the back plate 7,and the frame and diaphragm assembly laid carefully upon the backplate-7. The whole is then inverted, the clamping plate 8 positionedaround the back plate 7 and the supporting screws 10 inserted and drawnup to a suitable tension.

The bridge piece 26 is then assembled with the granule cup 21, itsconductors 22 and 23 and supporting member 241 and the clamp 25 inplace. The complete assembly is then laid upon the frame 1 withoutintervening insulation and the granule cup 23 on its supporting rod 24moved downwardly until the edges of the granule cup 23 are in contactwith the diaphragm 3. The clamp 25 is next tightened to hold the rod 24firmly in place. The bridge member 21 ma then be invert 6 ed and asufiicient number 0 granules 31 put bridge piece.

into the cup 23 to fill'it almostfull. The insulating pieces 28, whichmay desirably be mica having a thickness between 0.001 and 0.002 inch,are then laid upon the ends. The frame and diaphragm assembly are thenturned over and lowered carefullyupon the The clamping screws 27 andinsulation 28 are then inserted and drawn up tightly. Theconstructionand assembly, as indicated, provide a spacing between the granule cupsand the diaphragm 3 which is less than the radius of the granules 6.

The damping action of the back plate 7 prevents the excursion of thediaphragm be ing greater than the spacing from the edges of the granulecups, and, therefore, there is no tendency for the granules to escapearound the edges of the cup. Likewise the'fact that the granule cupshave no rough edges, but instead only a smooth simple surface, proridesa support upon which the granules lie in a simple, symmetricalarrangement providing a simple pressure system, responsive to pressurevariation, to produce resistance variations which show a characteristiccurve free from random irregularities.

' The diaphragm is stretched by means of the rings 5 and 6 to such atightness that its natural period of vibration is high, preferablybeyond the limit of the frequencies with which the diaphragm is intendedto deal. Because the diaphragm is stretched, there is a tendency for itto respond more readily to the higher frequencies. In order to overcomethis, the thin air space between the diaphra gm and the back plate 7 issupplied.

lVheu the diaphragm moves toward the back plate, the air in the centralportion of this space can escape only by flowing radially until itreaches the holes 19. If the movement of the diaphragm is slow, the airhas ample time to escape by flowing radially to the holes and emergingthrough them. If the motion of the diaphragm is rapid, the air will becompressed before it can escape and will react against the diaphragm,thereby tending to damp its movement. When the diaphragm moves in theopposite direction, similar considerations, except that the air israrefied instead of-being compressed, apply and show that the rapidmovement of the diaphragm is clamped, although the slow movement is not.The action of the back plate 7, therefore, is a selective damping,

the higher frequencies being damped more than the lower frequencies. Thedamping thereby tends to counteract the influences of the stretching andthe two together produce a very uniform response.

By means of the construction embodied in my invention, I am enabled toprovide a microphone device in which the gross simple I departures fromrectilinear translation by the resistor granules is compensated bydeparture in opposite directions by two resistors and in which thetendene to respond differently to sounds having igh or low frequenciesis corrected and in which random irregularities of response by thegranule sys- I for, said means having a center recess adapttems isavoided.

While I have shown but a singleembodi ment. of my invention in thepreceding description, it is capable of various changes andmodifications therefrom without departing from the spirit thereof, andit is desired, therefore, that only such limitations shall be imposedthereon as are required by the prior art or indicated by the appendedclaims.

I claim as my invention:

1. A microphone comprising a diaphragm,

'stretching means and damping means therelit for, a supporting framecooperating with both thereof, resistor cups and granules supportedthereby, positioned on opposite sides of said diaphragm and spacedtherefrom by a distance less than the radius of said granules. 4

2. Amicrophone comprising a diaphragm,

. stretching means and damping means causing said diaphragm to have alow excursion in response to sounds of a given intensity, resistor cupshaving square edges and spaced from said diaphragm a distance less thantwice the excursion, and granules therein having a radius greater thantwice the excursion.

3. A microphone comprising a diaphragm,

means for forming a damping chamber thereed to contain resistorgranules, and a second granule container on the opposite side of saiddiaphragm.

4. A microphone comprising a diaphragm, a'frame therefor, and anon-conductive damping back plate having a granule recess therein, andholes therethrough adapted to control the ratio :of damping ofrelatively high and relatively low frequencies.

5. In a microphone, a diaphragm, a damping means therefor comprising aplate closely adjacent to said diaphragm and having openings near itsperiphery only and having a recess near its center andpressure-responsive resistance material in said recess.

6. In a microphone, a diaphragm, a damp-' ing means therefor comprisinga plate of insulating material closely adjacent to said dia- I phragmand having openings near its periphery only and h'avlng a recess nearits center and pressure-responsive resistance material in said recess.

In testimony whereof, I have hereunto subscribed my name this 24th dayof June, 1925.

- JOSEPH EUGENE AIKEN.-

